1. Field of the Invention
The present invention relates to an optical material and optical element, diffraction optical element, and stacked type diffraction optical element molded of the optical material.
2. Description of the Related Art
Conventionally, refraction optical systems which use refraction of light to reduce chromatic aberration do so by using a combination of lenses made of glass material with different dispersion characteristics. For example, in a telescope, chromatic aberration which occurs on an axis of an object lens is corrected using a combination of positive lenses made of high-dispersion glass material and negative lenses made of low-dispersion glass material. However, it is sometimes difficult to correct chromatic aberration sufficiently if there are limits on the configuration or number of lenses or on available glass materials.
A. D. Kathman and S. K. Pitalo disclose in “Binary Optics in Lens Design.” International Lens Design Conference, 1990, SPIE Vol. 1354, pp. 297-309 that the combined use of refractive optical elements which have a reflection plane and diffraction optical elements which have a diffraction grating allows chromatic aberration to be reduced using a small number of lenses.
This uses a physical phenomenon in which chromatic aberrations caused by a light beam of a reference wavelength are opposite in direction between a reflection plane and refraction plane of an optical element. Also, if a period of a diffraction grating continuous with a diffraction optical element is changed, characteristics equivalent to those of an aspherical lens are available.
However, a light beam incident on the diffraction optical element is divided by diffraction effects into multiple lights of different orders. At this time, diffracted lights of orders other than a designed order are focused on locations different from light of the designed order, causing flare.
U.S. Pat. No. 5,847,877 discloses a technique for achieving high diffraction efficiency over a wide range of wavelengths by optimizing optical dispersion of optical elements and shape of a grating formed on boundaries among the optical elements. The technique concentrates light fluxes in a usable wavelength range on a particular order, thereby reducing intensities of irrelevant diffracted lights (designed order) and thereby preventing flare.
U.S. Pat. No. 5,847,877 also discloses combined use of diffraction optical elements made of a material with optical scatteringlow dispersion and diffraction optical elements made of a material with optical scatteringhigh dispersion to provide a configuration which has high diffraction efficiency over a wide range of wavelengths.
That is, the larger the difference in optical dispersion between a material with optical scatteringlow dispersion and material with optical scatteringhigh dispersion, the higher the diffraction efficiency of the resulting optical element and the wider the field angle of the optical element. Thus, to correct chromatic aberration with high accuracy, it is necessary to combine a material having higher dispersion (smaller Abbe's number) with a material having lower dispersion (larger Abbe's number).
U.S. Pat. No. 6,912,092 discloses an optical material wherein a relationship between a refractive index (nd) and Abbe's number (νd) with respect to line n is given by nd>−6.667×10−3νd+1.70 and a relationship between a secondary dispersion (θg,F) and the Abbe's number (νd) is given by θg,F≦−2νd×103+0.59. If the equations are satisfied, the diffraction efficiency can be improved over an entire visible region. The optical material disclosed in U.S. Pat. No. 6,912,092 is a compound material produced by mixing and dispersing fine particles in an organic resin serving as a base. The fine particles are made of transparent conductive metal oxide with high dispersion and low secondary dispersion characteristics. Possible transparent conductive metal oxides which are disclosed include ITO, ATO, SnO2, and ZnO.
However, with a compound material of fine particles and resin, optical scattering occurs at interfaces between the fine particles and resin. Generally, optical scattering (Rayleigh scattering) increases in direct proportion to the sixth power of particle size, the fourth power of the inverse of wavelength, and a reflection coefficient (a ratio of a refractive index). Fine particles of metal oxides such as ITO often have a larger reflection coefficient than organic resin which serves as a base, resulting in a large ratio of reflection coefficient between the metal oxides and base. Consequently, it is necessary to reduce the optical scattering.
With a compound of fine particles of a metal oxide and organic resin which serves as the base, to reduce optical scattering, it is necessary to reduce the particle size at least below the wavelength of the light to be used. However, agglomeration of fine particles of metal oxides increases with decreases in the particle size due to van der Waals forces. This increases apparent particle size, resulting in intense optical scattering.
Furthermore, to reduce optical scattering, it is desirable to minimize particle size distribution of each type of fine particle and to make physical properties of each type uniform whenever possible. However, manufacture of fine particles of metal oxides is very demanding. Slight differences in manufacturing conditions can cause oxygen deficiency in the metal oxides and affect the degree of crystallinity. These changes in turn affect the shape and physical properties of the fine particles, making it very difficult to keep the particle size and physical properties of the fine particles uniform.